1. Field of the Invention
The present invention relates to a method and the corresponding apparatus used to determine the wax appearance temperature or cloud point of waxy or paraffinic petroleum oils. More specifically, the present invention relates to a method to determine the wax appearance temperature of paraffinic petroleum oils by measuring the change in density of the petroleum oil at the wax appearance temperature by means of a Pressure-Volume-Temperature (PVT) cell coupled to a high pressure and temperature densimeter.
Throughout the present specification, the terms "petroleum oils" and "fluid" represent hydrocarbon fluids as present in the reservoir, with or without gas dissolved therein.
Generally, the composition of oils and hydrocarbon fluids is of such complexity that it is difficult to estimate the number of components of such fluids. However, it is usual to consider the presence of straight-chained (n-paraffins) and branched paraffins (iso-paraffins), naphthenes (cycloparaffins) and aromatic compounds. Besides, there are further small amounts of compounds such as asphaltenes and resins which include heteroatoms and heavy metals.
It is also important to distinguish between dead oil (without gas) and live fluid (with gas, such as occurs in a reservoir). The content of light hydrocarbons (C.sub.1 to C.sub.6) in live fluid is in the order of 20 mol % higher than for dead oil.
For the straight-chained paraffins, the change in physical properties is proportional to the increase in chain length. The branched paraffins have less predictable properties. An increasing level of branching will, in most cases, lead to a reduction in the boiling point and melting point. The composition of paraffins in each oil is a function of factors such as the geological origin of the oil. For wax appearance this means that it is very difficult to characterize the material appearing. The wax which appears will mainly be normal paraffins on account of the higher content of these components and because the melting points for n-paraffins are considerably higher than for most other components in the oil. Models which are based on a wax fraction which is dissolved in the rest of the oil must therefore have a good analytical description of the composition of the complete fluid, in both its solid and liquid phases.
It is common knowledge that the content of light components in the fluid exerts an influence on the solubility of the longer chain components at a given temperature. Besides, the increase in pressure, as a function of a higher content in light components, will affect the properties of the components in the oil and thus also the solubility. Thus the overall effect of higher pressure and higher content in light components will depend on the total composition of the fluid.
On the other hand, the improvement in the exploration of paraffinic oil reservoirs necessarily involves a mathematical model which would represent the behavior of such oils as concerns their flow in porous media and pipelines as well as the thermodynamic balance of the gas, liquid and solid phases, the latter phase having origin in the paraffins which crystallize out from solution.
The development of the model requires that the behavior of the fluid in some of the conditions where the real phenomenon occurs in the reservoir or in the pipelines, be known. Once this behavior is adjusted, the model can be applied to effect simulations and determine the fluid behavior in any condition.
To determine the wax appearance point or cloud point of an oil at different compositions, the influence of pressure on the wax appearance point and a study on the reversibility of the process are of paramount importance for the modelling of a paraffinic oil or a hydrocarbon fluid.
2. Description of the Related Art
According to the ASTM Standard Test No. D2500-91 the wax appearance point is determined by the direct visualization of the formation of waxy or paraffinic crystals in the fluid within a transparent vessel kept in a controlled temperature bath. Because a mist or cloud is developed in the fluid, the wax appearance point is also known as its cloud point. The above mentioned ASTM Standard is limited to clear fluids.
In the technique known as Differential Scanning Calorimetry, or DSC, the measurement of the heat released during the solidification of the paraffin crystals is the basis for determining the cloud point. However, this technique can yield lower than real figures for wax appearance point, mainly in the case of petroleum oils of low paraffin content.
In the optical microscopy process, the measurement of the wax appearance point is obtained by detecting the appearance of the paraffin crystals on the thin layer of an optical microscope. This thin layer is coupled to a system of controlled cooling. The polarized light reaching the thin layer renders easier to detect the onset of wax appearance. The preparation of a thin layer (50 micrometers) and the incidence of light render it possible to use opaque fluids such as petroleum oils. While the optical microscopy method provides wax appearance points with reasonable accuracy, it is not a very practical method as an everyday, routine analytical tool.
In another approach, the viscosity is determined at several temperatures. The cloud point is identified by an inflection on the viscosity vs. temperature curve. As in the DSC technique, this technique can equally yield figures for wax appearance point which are inferior to the real ones.
GB-A-2268276 describes a method of determining the wax appearance point in a petroleum product wherein the petroleum volume is measured and plotted as a function of temperature at constant pressure, wherein the wax appearance point of the waxy or paraffinic phase occurs at the temperature at which there is a deviation in the graph. The method described in GB-A-2268276 comprises further determining the quantity of wax in a petroleum product at a particular temperature below the wax appearance point by (i) estimating the density of the solid and liquid wax, (ii) measuring the change in volume of the petroleum product due to the formation of solid wax phase, and (iii) multiplying the difference of the density of the wax in the two phases as determined by the measured volume change.
According to British GB-A-2268276 phase changes in the fluid derive from a temperature variation. The alleged advantage lies in the fact that the measurement of certain fluid properties can be directly explained by the transition from liquid phase to solid phase. The apparatus used in the method of the British publication comprises a pressure cell placed in a thermostatic bath, a pump connected to the pressure cell to produce pressure, sensors to determine pressure, temperature and volume in the cell and a control unit to read and set the cell pressure, temperature and volume.
However, it has been found that, since the accuracy of volumetric measurements by pumps such as the ones described in GB-A-2268276 is of 0.01 cm.sup.3, then for a 60 cm.sup.3 sample described in FIG. 2 of the publication, the lowest detectable volume contraction is (0.01 cm.sup.3 in 60 cm.sup.3)=0.017%, this order of magnitude being present only for the wax appearance point of highly paraffinic oils. Thus, the method described in GB-A-2268276 is in practice limited to highly paraffinic oils. Besides, there are drawbacks involved in preparing several samples of 60 cm.sup.3 each, having varying amounts of gas, in order to vary the oil composition and thus determine the influence of the composition on the wax appearance point. In order to determine the influence of the composition on the wax appearance point it is necessary to have at least 1000 cm.sup.3 of oil which must be solubilized with varying amounts of gas.
Therefore, there is the need for a method and high-precision apparatus to determine the wax appearance point of oils with a wide range of paraffinic contents, including contents as low as 0.5 wt %, using small volume samples, the measurements being effected in short periods of time, these advantages being presented by the present application.